On Thu, 16 Jul 1998 Philip.Belben_at_powertech.co.uk wrote:
> > When a phone is on-hook (hung-up) there is a 48V potential on the line.
> > When the phone is ringing, there is 90v (nominal) AC on the line during
> > the ring cycle to activate the ringer. When a phone is off-hook (you're
> > talking) there is 12v on the line.
>
> I think the 48V on hook and 12V off hook is historical - in the days of
> long copper wires back to the exchange, it was all done from the 48V
> battery - 12V was all that was left after voltage drop when the 30mA or
> so (apparently the carbon microphone needs at least 23mA to work well)
> had to go the several miles to your house and back. May still be like
> that, come to think of it.
The voltages must remain the same in order to be compatible with old
equipment per government regulations. Believe it or not there are still
many people out there with old (ancient) rotary dial telephones. The part
about the voltage drop definitely applies still today as you're looking at
several hundred ohms of resistance in the "subscriber loop" (the loop
from the telco to the home).
> Those on shorter lines had higher resistance phones to compensate. In
> the 1960s (? What date is the 706 anyway ?), many UK phones had the
> "line drop compensator" on a plug-in module, presumably so you could
> swap it for one with different resistors...
Interesting. I've never heard of this, but then you Euro folks do things
oddly over there :)
Sam Alternate e-mail: dastar_at_siconic.com
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Received on Thu Jul 16 1998 - 16:12:58 BST